A gamma-ray burst detected by ESA's Integral gamma-ray observatory on 3
December 2003 has been thoroughly studied for months by an armada of
space and ground-based observatories. Astronomers have now concluded
that this event, called GRB 031203, is the closest cosmic gamma-ray
burst on record, and also the faintest. This also suggests that an
entire population of sub-energetic gamma-ray bursts has so far gone
unnoticed.

Cosmic gamma-ray bursts (GRBs) are flashes of gamma rays that can last
from less than a second to a few minutes and occur at random positions
in the sky. A large fraction of them is thought to result when a black
hole is created from a dying star in a distant galaxy. Astronomers
believe that a hot disc surrounding the black hole, made of gas and
matter falling onto it, somehow emits an energetic beam parallel to the
axis of rotation.

According to the simplest picture, all GRBs should emit similar amounts
of gamma-ray energy. The fraction of it detected at Earth should then
depend on the 'width' (opening angle) and orientation of the beam as
well as on the distance. The energy received should be larger when the
beam is narrow or points towards us and smaller when the beam is broad
or points away from us.

New data collected with ESA's high energy observatories, Integral and
XMM-Newton, now show that this picture is not so clear-cut and that the
amount of energy emitted by GRBs can vary significantly. "The idea that
all GRBs spit out the same amount of gamma rays, or that they are
'standard candles' as we call them, is simply ruled out by the new
data," said Dr Sergey Sazonov, from the Space Research Institute of the
Russian Academy of Sciences, Moscow (Russia) and the Max-Planck
Institute for Astrophysics, Garching near Munich (Germany).

Sazonov and an international team of researchers studied the GRB
detected by Integral on 3 December 2003 and given the code-name of GRB
031203. Within a record 18 seconds of the burst, the Integral Burst
Alert System had pinpointed the approximate position of GRB 031203 in
the sky and sent the information to a network of observatories around
the world. A few hours later one of them, ESA's XMM-Newton, determined a
much more precise position for GRB 031203 and detected a rapidly fading
X-ray source, which was subsequently seen by radio and optical
telescopes on the ground.

This wealth of data allowed astronomers to determine that GRB 031203
went off in a galaxy less than 1300 million light years away, making it
the closest GRB ever observed. Even so, the way in which GRB 031203
dimmed with time and the distribution of its energy were not different
from those of distant GRBs. Then, scientists started to realise that the
concept of the 'standard candle' may not hold. "Being so close should
make GRB 031203 appear very bright, but the amount of gamma-rays
measured by Integral is about one thousand times less than what we would
normally expect from a GRB," Sazonov said.

A burst of gamma rays observed in 1998 in a closer galaxy appeared even
fainter, about one hundred times less bright than GRB 031203.
Astronomers, however, could not conclusively tell whether that was a
genuine GRB because the bulk of its energy was emitted mostly as X-rays
instead of gamma-rays. The work of Sazonov's team on GRB 031203 now
suggests that intrinsically fainter GRBs can indeed exist.

A team of US astronomers, coordinated by Alicia Soderberg from the
California Institute of Technology, Pasadena (USA), studied the
'afterglow' of GRB 031203 and gave further support to this conclusion.
The afterglow, emitted when a GRB's blastwave shocks the diffuse medium
around it, can last weeks or months and progressively fades away. Using
NASA's Chandra X-ray Observatory, Soderberg and her team saw that the
X-ray brightness of the afterglow was about one thousand times fainter
than that of typical distant GRBs. The team's observations with the Very
Large Array telescope of the National Radio Astronomy Observatory in
Socorro (USA) also revealed a source dimmer than usual.

Sazonov and Soderberg explain that their teams looked carefully for
signs that GRB 031203 could be tilted in such a way that most of its
energy would escape Integral's detection. However, as Sazonov said, "the
fact that most of the energy that we see is emitted in the gamma-ray
domain, rather than in the X-rays, means that we are seeing the beam
nearly on axis." It is, therefore, unlikely that much of its energy
output can go unnoticed.

This discovery suggests the existence of a new population of GRBs much
closer but also dimmer than the majority of those known so far, which
are very energetic but distant. Objects of this type may also be very
numerous and thus produce more frequent bursts.

The bulk of this population has so far escaped our attention because it
lies at the limit of detection with past and present instruments.
Integral, however, may be just sensitive enough to reveal a few more of
them in the years to come. These could be just the tip of the iceberg
and future gamma-ray observatories, such as the planned NASA's Swift
mission, should be able to extend this search to GRBs of much lower
energy, where many more of them are expected.